Degaussing technique

ABSTRACT

To degauss cassettes of magnetic tape, a magnetic field is applied to the magnetic material with a flux density of at least 1,000 gauss and at an angle between 20 degrees and 70 degrees from the horizontal of the magnetic tape for a time period of at least one second and the field is alternated at a frequency of at least ten hertz. The magnetic material may be rotated while it is in the field.

RELATED CASES

This application is a continuation-in-part of United States applicationSer. No. 07/870,476, filed Apr. 17, 1992, now U.S. Pat. No. 5,204,801,issued Apr. 20, 1994, in the names of Donald Gene Becker and DavidJoseph Etherton and assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatuses for erasinginformation from a magnetic recording medium.

In one class of methods and apparatuses for erasing information frommagnetic recording media, the recording medium, which may be a magnetictape wound about a reel, is subjected to a varying or alternatingelectromagnetic field to randomize the magnetic particles on themagnetic material.

In one prior art method and apparatus in this class, the magnetic tapeis moved into an electromagnetic field that is applied in each of aplurality of different directions, one direction at a time, such as byfirst applying a vertically oriented field followed by a .longitudinallyoriented field. Techniques of this type are described in U.S. Pat. Nos.4,730,230 and 4,751,608.

In another prior art technique, the magnetic tape is carried by aconveyor over a rotating electromagnet that has pole faces parallel toeach other in the same plane underneath the conveyor belt. Thus, theelectromagnet rotates a time varying electromagnetic field with it tocause the time varying electromagnetic field to pass through themagnetic material in the tape at a plurality of different angles. Thistype of prior art device is disclosed in U.S. Pat. No. 4,639,821.

Still another prior art apparatus and technique of this class includes aconveyor that carries a magnetic tape into a rotating magnetic field.The rotating magnetic field is substantially parallel with the conveyorand is in the plane of the magnetic tape. It is created byelectromagnetic poles on both sides of the conveyor belt, energized insuch a way that similar polarities oppose each other on opposite sidesof the tape with the phases of the poles on each side of the conveyorchanging in synchronism to cause the field to rotate. Thus, northelectromagnetic poles face each other on opposite sides of the tape andsouth magnetic poles face each other on opposite sides of the tape andthe north and south poles alternate with each other in the same plane onthe same sides of the tape. The poles rotate in synchronism.

The prior art degaussing techniques provide erasure that is satisfactoryfor some purposes but do not erase to the extent desired for otherapplications. In general, the systems which apply vertical andlongitudinal fields separately have the disadvantage of moving theenergy back and forth between even and odd harmonics of the recordedsignal. This reduces the effectiveness of the erasure. Rotational fieldsby themselves do not improve the depth of erasure to the extent neededfor some applications when practiced as described in the aforementionedprior art references.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a noveldegaussing apparatus and method.

It is a further object of the invention to provide a novel technique forapplying an alternating current, electromagnetic field to a magneticmedium.

It is a still further object of the invention to provide a technique forincreasing the depth of erasure of information recorded on a magneticmedium over other techniques for erasing recorded information from amagnetic medium without increasing the magnetic flux density.

In accordance with the above and further objects of the invention, amagnetic field vector is applied through a tape cassette at an angle ofbetween 20 to 70 degrees to the direction of the orientation of magneticdomains on the medium, or of course, the supplement of the angles inthis range. For the common longitudinally recorded magnetic tape, thefield vector is between 20 to 70 degrees from the longitudinal axis of astrip of the tape.

For a cassette having this orientation of recorded information, thefield may be applied at an angle of between 20 to 70 degrees to thelarger flat sides of the cassette. Generally, if this is done, the fieldis rotated about an axis that is perpendicular to the flat sides of thecassette so that all sections of the tape receive a vector with a properorientation at some time during the rotation of the field. Thealternating field is applied at a sufficient magnetic flux density tochange the orientation of individual magnetic domains or particles andthus randomize the orientation.

The field is applied parallel to a plane that is: (1) at an acute orobtuse angle to the longitudinal direction of the tape; and (2) at anobtuse or acute angle to the side-to-side direction of the tape. Thus,the field vector is not perpendicular to the direction of magnetization(which is generally along the longitudinal axis of the tape) nor is thefield vector in the direction of magnetization. However, it has verticaland longitudinal components that are simultaneously present at the samepoint in the tape.

The angle is selected so that when resolved into components, themagnetic flux density has a vertical component nearly equal to or lessthan that of the longitudinal component. Keeping the ratio of verticalcomponent to longitudinal component (the tangent of the vector angle)between 2.7 and 0.36 prevents energy transfer between even and oddharmonics of the recorded signal during degausing and results in evenerasure of all harmonics. Preferably, angle is selected so that thesignal fundamental and all harmonics are reduced below a desired degausslevel. In the preferred embodiment, the angle is approximately 45degrees from the horizontal and the reel is rotated as it is movedlinearly through the field.

It can be understood that the degausser of this invention has theadvantage of providing a greater amount of erasure with the .same fluxdensity than other techniques which apply fields in different directionsin different stages or apply them directly or predominantly in thedirection of magnetization or normal to the direction of magnetization.

SUMMARY OF THE DRAWINGS

The above noted and other features of the invention will be betterunderstood from the following detailed description when considered withreference to the accompanying drawings in which:

FIG. 1 is a fragmentary, side elevational view of a degaussing apparatusand tape to be degaussed in accordance with an embodiment of theinvention;

FIG. 2 is another fragmentary, side elevational view of the degaussingapparatus of FIG. 1 in another state of its progression duringdegaussing;

FIG. 3 is a top view of the embodiment of FIG. 1;

FIG. 4 is a fragmentary, side elevational view of another embodiment ofthe invention; and

FIG. 5 is a top view of the embodiment of FIG. 3.

DETAILED DESCRIPTION

In FIGS. 1 and 2, there is shown a fragmentary, simplified sectionalview of a degausser 10 in the process of receiving and degaussing amagnetic tape cassette 12 containing magnetic tape for degaussing. Thedegausser 10 includes a source of a magnetic field 16, a conveyorassembly 14 for delivering the magnetic tape cassette 12 into themagnetic field 16, and a system shown at 18A and 18B for causing thefield 16 and/or the cassette 12 to rotate with respect to each other.FIG. 1 shows the tape 12 entering the degaussing apparatus and FIG. 2shows the tape 12 within the field.

The source 16 of the magnetic field is positioned with respect to theconveyor so that when the cassette 12 is moved into the field, thesource 16 creates a field at an angle to the side of the cassette 12that is in the range of 20 to 70 degrees, and in the preferredembodiment, 45 or fewer degrees. This angle is referred to in thisspecification as the "effective angle". FIG. 1 shows one effective anglefrom one side and FIG. 2 shows a different effective angle that is thesupplement of the effective angle shown in FIG. 1. It is desirable forthe domains of the tape to receive peak or substantially a peak fieldstrength at both the effective angle and its supplement.

In using the effective angle, a magnetic field vector is applied throughthe tape at an angle of between 20 to 70 degrees from the longitudinalside edges of the tape. The alternating field is applied at a sufficientmagnetic flux density to change the orientation of individual magneticdomains or particles and thus randomizes the orientation.

The field is applied parallel to a plane that is: (1) at an acute orobtuse angle to the longitudinal direction of the tape; and (2) at anobtuse or acute angle to the side-to-side direction of the tape. Thus,the field vector is not perpendicular to the direction of magnetization(which is generally along the longitudinal axis of the tape) nor is thefield vector in the direction of magnetization. However, the fieldvector has components that are simultaneously present in theperpendicular and longitudinal directions.

The effective angle is selected so that when resolved into components,the magnetic flux density has a vertical component nearly equal to orless than that of the longitudinal component. More generally, the ratioof the vertical component to the longitudinal component (the tangent ofthe vector angle) is preferably between 2.7 and 0.36. This reducesenergy transfer between even and odd harmonics of the recorded signalduring degausing and results in even erasure of all harmonics. The angleis selected so that during degaussing, the fundamental and all harmonicsare reduced below a desired level.

In the preferred embodiment, the angle is approximately 45 degrees fromthe horizontal and the reel is rotated as it is moved linearly throughthe field. At 45 degrees, the vertical and longitudinal field componentsare in equal proportions at a given point in space. Unlike otherdegaussers in which the magnetic field is applied at a number of anglesbut at different points in space or time resulting in ineffectivedegaussing of all harmonics without consideration of the best angle, thedegausser 10 applies the magnetic vector at the effective anglepreferably all of the time, but at least a sufficient proportion of thetime the degausser operates on the magnetic material to be efficientsuch as for example at least 30 percent of the time the tape is in thefield.

Of particular significance is that the field be at the effective anglefor the domains to be erased for at least one and one-half cycles andthat it be at the effective angle during at least one peak of thealternating current as the tape and field are separated such as when acassette is moved from a field by a conveyor. The reduction of the fieldstrength from the fringes of the field at the time the tape and fieldare being separated is particularly effective in erasing the tape.

The conveyor assembly 14 in the embodiment of FIG. 1 includes a belt 20,a first roller 22 and a second roller 24. Either or both of the rollersare drive rollers with the belt 20 passing over them as an endless beltcapable of moving the cassette 12 or a series of such cassettes alongthe top run of the belt 20. Of course, any other means for moving thecassettes with respect to the field may be used.

The source of the magnetic field 16 includes first and secondelectromagnetic assemblies 30 and 32, respectively. One of theassemblies 32 in the embodiment of FIG. 1 is located close to the toprun of the belt 20 and beneath the belt 20 so that the cassette 12passes over it and the other electromagnetic assembly 30 is locatedabove the top run of the belt 20 at an elevation high enough so that thecassette 12 may pass therebetween.

The first electromagnetic assembly 30 includes a first coil 34, a firstcore leg 36, a bridging portion 38, a second coil 40 and a second coreleg 42. The first and second coils 34 and 40 are respectively woundaround the first and second downwardly extending vertical legs 36 and 42to create a field through them with the bridge 38 connecting the legs 36and 42 to provide a closed magnetic path, to hold them in position, andto mount them to the means for rotating 18B in fixed relationship withrespect to each other. The first core leg 36 ends in a pole face 54 andthe second core leg 42 ends in a pole face 56. The legs 36 and 42 are ofhigh permeability metal to concentrate the field at the pole faces 54and 56 for extension downwardly through a cassette 12 to the secondelectromagnetic assembly 32.

The second electromagnetic assembly 32 is constructed in a mannersimilar to the first assembly 34 and includes a first coil 44, a firstupwardly extending, core leg portion 46, a bridge 48, a second coil 50and a second upwardly extending, core leg portion 52. The first leg 46ends in a pole face 58 and the second leg 52 ends in a pole face 60. Thecore legs 36, 42, 46 and 52 are parallel to each other with the polefaces 58 and 60 being positioned near the bottom of the bottom rung ofthe belt 20 and the pole faces 54 and 56 being located at a sufficientheight so that the cassette 12 passes under them when carried intoposition by the belt 20.

The pole faces of opposite polarity on opposite sides of the top run ofthe belt 20 are off-set from each other so that a field extendingbetween the pole face 54 and 58 and the field extending between the poleface 56 and 60 are at an angle to the cassette 12. That angle is between20 and 70 degrees so that if the components of the field are resolvedinto components that are vertical to the cassette 12 and horizontal toit, the vertical component would be 0.36 to 2.7 times the magnitude ofthe longitudinal component. Thus, the critical angle is formed for themagnetic vector.

The bridges 38 and 48 are preferably of high permeability material andthe windings wound in a direction so that the poles alternate north tosouth. In this case, the poles 58 and 60 are of opposite polarity andthe pole 58 has the opposite polarity from the pole 54 so that if 54 isnorth, 58 is south. In this embodiment, the reluctance path between thepoles 54 and 56 and the reluctance of the path between the poles 58 and60 should be much greater than the reluctance of the path between thepoles 54 and 58 and the poles 56 and 60. This may be accomplished byspacing the poles 54 and 56 and the poles 58 and 60 much further fromeach other than the poles 54 and 58 and the poles 56 and 60 are spacedfrom each other.

The rotating system portions 18A and 18B are identical to each other andeach includes a motor driven shaft which serves to rotate theelectromagnetic assemblies in synchronism to maintain the effectiveangle between the poles 54 and 58 and 56 and 60. For this purpose, theshaft 19B and 19A are connected off-center to the bridges 38 and 48,respectively, to support the bridges and thus, the core legs andwindings as they are rotated.

The shafts 19A and 19B are parallel to each other and perpendicular tothe bridges 48 and 38 so that the pole faces 54, , 56, 58 and 60 remainparallel to the belt 20. Power in this embodiment is electricallyconnected to the windings through slip rings 21B and 21A connected to asource of A.C. power such as the source 23 shown connected to the sliprings 21B with the shaft 19B adapted to slide with respect to the sliprings and the slip rings being electrically connected to the windings.

In FIG. 3, there is shown a simplified top view of the embodiment ofFIG. 1 showing the cassette 12 being carried by the belt 20 into aposition where it is between the top positioned coils 34 and 40 and thebottom coils 44 and 50. The windings and conveyor are conventional. Thebelt 20 and electromagnets 30 and 32 may be fabricated in the mannerdescribed in connection with U.S. Pat. No. 4,897,759.

Either the cassette 12 or the electromagnetic assemblies 30 and 32 maybe rotated since it is the motion between the two that is significantand similarly the tape 12 may be moved between the assembly or theassembly moved over the tape 12. Moreover, instead of physicallyrotating the electromagnets 30 and 32, a rotating magnetic field may becreated, such as that disclosed in U.S. Pat. No. 4,423,460, provided theangular direction of the field as it passes through the cassettes ismaintained at the effective angle.

In FIG. 4, there is shown a simplified elevational sectional view ofanother embodiment 10A of degausser positioned to erase a cassette 12having a means for moving the cassette 12 and degaussing field withrespect to each other for erasing information from the magnetic tape, ameans for rotating the tape with respect to the field, and a source of amagnetic field adapted to be applied at an angle to the tape forefficient demagnetization thereof. In this embodiment, the means formoving the cassette and degaussing field with respect to each other issimilar to a file cabinet drawer 74 positioned to move with respect to asource of a magnetic field. The drawer 74 in this embodiment includes adrawer door 82, a drawer frame 84 and a drawer roller assembly 86.

The drawer frame 84 supports a rotating means 80 and is connected to thedoor 82 for moving on the drawer rail and roller assembly 86. The sourceof the magnetic field 16 (not shown in FIG. 3) is mounted to bestationary with respect to the frame 84 so that the means for rotatingthe cassette 12 is moved between the first and second electromagneticassemblies 30A and 32A for demagnetization.

A means 80 for rotating the cassette 12 includes a pan mounted forrotation within the frame 84 by bearings 87 such as along the rim of acircular opening 89 in the bottom of the frame 84. To rotate thecassette 12, a drive motor 90 is connected by a belt 92 to the rim ofthe pan 96 for rotating about the idler pulley 94 so as to turn the pan.The tape mounts within the pan 96 and is rotated therewith within thefield 16. The first and second electromagnetic assemblies 30A and 32Aare mounted to the side of the cabinet by frame members 70 and 72 (notshown in FIG. 3).

In FIG. 5, there is shown a plan view of the degausser 10A showing themanner in which cassette 12 is mounted for rotation with the pan 96. Thepan 96 as best shown in FIG. 4, is positioned to be rotated by the motor90, idler pulley 94 and belt 92 to rotate the cassette 12. Theelectromagnetic assemblies 30A and 32A are identical to those in theembodiment of FIGS. 1 and 2 except that they are mounted to bestationary rather than rotatable.

In each of these embodiments, the tapes may be rotated or not. Slightlybetter demagnetization is obtained by rotating the tapes, probablybecause the intensity of the field at the effective angle is evenlydistributed by the rotation over the tape around the reel during therotation. Otherwise, the orientation of the tape within the field couldcause some portions to receive less activation than others.

In operation, a cassette or other holder for magnetic tape, is placedeither on the conveyor belt 20 in the embodiments of FIGS. 1 and 2 or inthe pan 96 in the embodiments of FIGS. 3 and 4. The cassette, orplurality of cassettes, are then moved linearly into an alternatingmagnetic field which alternates at a frequency of approximately 60 hertzin a field of magnitude proportional to the magnetic coercivity of thecassette tape of other medium to be degaussed. Satisfactory results,however, have been obtained for a reduction of 95 decibels belowsaturation of a 750 oersted coercivity tape using a field vector of only2700 gauss.

While a 60 hertz field is generally used, other frequencies may be usedwithin the range of 10 to 400 hertz. The magnetic flux density should beat least two times the tape or medium coercivity. For todays media, theminimum field should be about 1,000 gauss. Generally the higher thefield, the better the depth of eraser, but other factors such asinductive heating effects and power loss that increase with higher fluxlimit the field strength in practical designs.

The tape is rotated in the rotating pan or the magnet assemblies arerotated. The rate of rotation of the tapes, when used, may be as low as40 revolutions per minute or higher. The linear movement of the tape,such as on a conveyor belt with a plurality of tapes being moved, may beat any convenient speed such as 0.3 inches per second but should be in arange of 0.1 inches per second to two inches per second. The rates ofrotation and linear speed are interdependent and are selected prior tothe degaussing in conjunction with the depth of erasure needed, theintensity of field that is to be applied and the coercivity of themagnetic medium.

From the above description, it can be understood that the degausser ofthis invention has several advantages, such as: (1) it is relativelyeconomical and fast in operation because it applies a single field at aneffective angle; and (2) it provides a greater level of erasure for thesame magnetic flux density than other techniques.

Although a preferred embodiment of the invention has been described withsome particularity, many modifications and variations in the inventionare possible in the light of the above teachings. Therefore, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced other than as specifically described.

What is claimed is:
 1. A method of degaussing magnetic materialcomprising the steps of:applying a magnetic field to the magneticmaterial with a flux density of at least 1,000 gauss and at an anglebetween 20 degrees and 70 degrees; and alternating the field at afrequency of at least ten hertz for a sufficient time so that the fluxdensity of at least 1,000 gauss passes through each portion of themagnetic material having a signal recorded on it at the angle of between20 degrees and 70 degrees for at least one and one-half cycles of thealternating field.
 2. A method in accordance with claim 1 furtherincluding the step of rotating at least one of the magnetic material andthe field with respect to each other while the magnetic material is inthe field.
 3. A method in accordance with claim 2 further including thestep of moving a plurality of cassettes of magnetic material insuccession through the field.
 4. A method according to claim 1 furtherincluding the steps of positioning pole faces of a source of magneticflux so that the flux path between them is at the angle of between 20degrees and 70 degrees with respect to the magnetic material.
 5. Amethod according to claim 4 in which the pole faces are stationary withrespect to each other and the magnetic medium is moved, with respect tothe pole faces while maintaining an angle of the peak field between thepole faces between 20 degrees and 70 degrees with respect to themagnetic medium.
 6. A method according to claim 1 in which the magneticfield and magnetic medium are rotated with respect to each other whilemaintaining an angle of the peak field density of between 20 degrees and70 degrees to the magnetic material.
 7. A method according to claim 1 inwhich a signal is recorded on the magnetic medium prior to degaussing,and the method of degaussing reduces energy transfer between even andodd harmonics of the recorded signal during degaussing whereby afundamental and all harmonics are reduced on the magnetic medium below apredetermined level.
 8. A method in accordance with claim 1 furtherincluding the step of moving a plurality of cassettes of magneticmaterial in succession through the field.
 9. A method in accordance withclaim 8 where in the field is at an angle of 45 degrees to the magneticmaterial.
 10. Apparatus for degaussing magnetic materialcomprising:means for applying a magnetic field to the magnetic materialwith a flux density of at least 1,000 gauss and at an angle between 20degrees and 70 degrees with respect to the magnetic medium; and meansfor alternating the field at a frequency of at least ten hertz for asufficient time so that the flux density of at least 1,000 gauss passesthrough each portion of magnetic material having a signal recorded on itat the angle of between 20 degrees and 70 degrees for at least one andone-half cycles of the alternating field.
 11. Apparatus in accordancewith claim 10 further comprising means for rotating the magneticmaterial while it is in the field.
 12. Apparatus in accordance withclaim 11 further comprising means for moving a series of magneticcassettes into the field while maintaining the field at an effectiveangle.
 13. Apparatus in accordance with claim 12 in which the means formoving includes a conveyor assembly.
 14. Apparatus according to claim 10in which the means for applying a magnetic field includes pole faces ofa source of magnetic flux so that the flux path between them is at theangle of between 20 degrees and 70 degrees to the magnetic material. 15.Apparatus according to claim 14 in which the pole faces are stationarywith respect to each other and further including means for moving themagnetic medium with respect to the pole faces while maintaining anangle of the peak field between the pole faces between 20 degrees and 70degrees with respect to the magnetic medium.
 16. Apparatus according toclaim 10 further including means for rotating the magnetic field andmagnetic medium with respect to each other while maintaining an angle ofthe peak field density of between 20 degrees and 70 degrees to themagnetic material.
 17. Apparatus according to claim 10 wherein there isa recorded signal on the magnetic medium about to be degaussed, and theapparatus for degaussing reduces energy transfer between even and oddharmonics of the recorded signal during degaussing whereby a fundamentaland all harmonies are reduced on the magnetic medium below apredetermined level.
 18. Apparatus according to claim 10 in which:themeans for applying include at least first, second, third and fourth polefaces; said first and second pole faces being connected through a firstlow reluctance path and being separated from each other by a first airgap; said third and fourth pole faces being connected through a secondlow reluctance path and being separated from each other by a second airgap; said first and third pole faces being separated by a third air gapshorter than each of said first and second air gaps; and means formoving said magnetic material and first and third pole faces withrespect to each other wherein said magnetic material and third air gapmovingly intersect.
 19. A method of degaussing magnetic materialcomprising the steps of:applying a magnetic field to the magneticmaterial with a predetermined flux density and at an angle selected toreduce strength of a recorded signal on the magnetic material below apredetermined level; and alternating the field at a frequency of atleast ten hertz for at least one and one-half cycles.
 20. A method inaccordance with claim 19 further including the step of rotating at leastone of the magnetic material and the field with respect to each otherwhile the magnetic material is in the field.
 21. A method in accordancewith claim 20 wherein the field is at an angle of 45 degrees to themagnetic material.
 22. A method according to claim 19 further includingthe steps of positioning pole faces of a source of magnetic flux so thatthe flux path between them is at the angle of between 20 degrees and 70degrees.
 23. A method according to claim 22 in which the pole faces arestationary with respect to each other and the magnetic medium is movedwith respect to the pole faces while maintaining an angle of the peakfield between the pole faces between 20 degrees and 70 degrees withrespect to the magnetic medium.
 24. A method according to claim 19 inwhich the magnetic field and magnetic medium are rotated with respect toeach other while maintaining an angle of the peak field density ofbetween 20 degrees and 70 degrees to the magnetic material.
 25. A methodaccording to claim 19 in which a signal is recorded on the magneticmedium prior to degaussing, and the method of degaussing reduces energytransfer between even and odd harmonics of the recorded signal duringdegaussing.
 26. A method according to claim 19 wherein the angle is onewithin a range of between 20 degrees and 70 degrees to the magneticmaterial.
 27. Apparatus for degaussing magnetic materialcomprising:means for applying a magnetic field to the magnetic materialwith a predetermined flux density and at an angle selected to reducestrength of a recorded signal on the magnetic material below apredetermined level; and means for alternating the field at a frequencyof at least ten hertz for at least one and one-half cycles. 28.Apparatus in accordance with claim 27 further comprising means forrotating the magnetic material while it is in the field.
 29. Apparatusin accordance with claim 27 further comprising means for moving a seriesof magnetic cassettes into the field while maintaining the field at aneffective angle.
 30. Apparatus in accordance with claim 29 in which themeans for moving includes a conveyor assembly.
 31. Apparatus accordingto claim 27 in which the means for applying a magnetic field includespole faces of a source of magnetic flux so that the flux path betweenthem is at the angle of between 20 degrees and 70 degrees.
 32. Apparatusaccording to claim 27 further including means for rotating the magneticfield and magnetic medium with respect to each other while maintainingan angle of the peak field density of between 20 degrees and 70 degreesto the magnetic material.
 33. Apparatus according to claim 27 in whichthe pole faces are stationary with respect to each other and furtherincluding means for moving the magnetic medium with respect to the polefaces while maintaining an angle of the peak field between the polefaces between 20 degrees and 70 degrees with respect to the magneticmedium.
 34. Apparatus according to claim 27 wherein there is a recordedsignal on the magnetic medium about to be degaussed, and the apparatusfor degaussing reduces energy transfer between even and odd harmonics ofthe recorded signal during degaussing whereby a fundamental and allharmonics are reduced on the magnetic medium below a predeterminedlevel.
 35. Apparatus according to claim 27 in which:the means forapplying include at least first, second, third and fourth pole faces;said first and second pole faces being connected through a first lowreluctance path and being separated from each other by a first air gap;said third and fourth pole faces being connected through a second lowreluctance path and being separated from each other by a second air gap;said first and third pole faces being separated by a third air gapshorter than each of said first and second air gaps; and means formoving said magnetic material and first and third pole faces withrespect to each other wherein said magnetic material and third air gapmovingly intersect; said selected angle being one in a range of between20 and 70 degrees with respect to the magnetic material.